Three terminal fuse-resistor device

ABSTRACT

A three terminal fuse-resistor protective device adapted for forming both a shunt connection and a low impedance series connection between the input and load portions of an electric circuit for protecting the circuit against overvoltage conditions and also against both low and high overcurrents. The overvoltage protective means is coupled between two terminals of the device which form the shunt connection. The overcurrent protective means are coupled between a different pair of terminals which define the low impedance series connection.

United States Patent [191 Nagel May 22,1973

1541 THREE TERMINAL FUSE-RESISTOR DEVICE [75] Inventor: Harry Henry Nagel, Westwood, NJ.

[73] Assignee: Bell Telephone Laboratories, Incorporated, Murray Hill, NJ.

[22] Filed: Dec. 30, 1971 [21] Appl. No.: 213,942

[52] US. Cl. ..337/164, 337/15, 337/182,

337/292 [51 Int. Cl. ..H0lh 85/04 [58] Field of Search ..3l7/40 A; 337/161,

[56] References Cited UNITED STATES PATENTS 3,628,093 11/1971 Crowley ..317/40 A 3,144,534 8/1964 Baumbach ...337/l65 X 2,400,408 5/1946 Haefelfinger 3 37/ 1 7 2,386,094 10/1945 Duerkob ..337/164 2,300,620 11/1942 Duerkob ..337/164 2,277,619 3/1942 Wallace ....337/163 X 2,111,749 3/1938 Bussmann ..337/164 Primary Examiner-Bemard A. Gilheany Assistant Examiner--A. T. Grimley Attorney-R. J. Guenther and William L; Keefauver [5 7 ABSTRACT A three terminal fuse-resistor protective device adapted for forming both a shunt connection and a low impedance series connection between the input and load portions of an electric circuit for protecting the circuit against overvoltage conditions and also against both low and high overcurrents. The overvoltage protective means is coupled between two terminals of the device which form the shunt connection. The overcurrent protective means are coupled between a different pair of terminals which define the low impedance series connection.

5 Claims, 3 Drawing Figures [8(FAST THERMAL RISE ELEMENT) |5(sL0w THERMAL RISE ELEMENT) Patented May 22, 1973 3,735,312

FIG. 2

'I Tf M 8 22 4 I6 I4 -l5 l8 2| 24 (SLOW THERMAL RISE ELEMENT) (FAST THERMAL RISE ELEMENT) FIG. 3

II 2 TERMINAL 2 TERMINAL I LOAD INPUT PORTION '26 )8 V 7 PORTION SHUNT PATH I5 LLEL EQ CIRCUIT TERMINAL 3 1 THREE TERMINAL FUSE-RESISTOR DEVICE BACKGROUND OF THE INVENTION This invention relates to fuse-resistor protective devices and, more particularly, to a three terminal fuseresistor protective device adapted for providing both a shunt connection and a low impedance series connection between the input and load portions of an electric circuit for protecting the circuit against overvoltage conditions and also against both low and high overcurrents.

Heretofore, fuse-resistor protective devices have been, in general, two terminal devices which are connected directly in series with the electric circuits that are to to protected. Such a device usually comprises a fusible element and a resistor which are both connected in series between the two terminals of the device. This construction places the impedance of the resistor in series with the circuit that is to be protected. In other words, the impedance of the resistor is added to the impedance of the circuit that is to be protected thus increasing the overall line impedance.

Although these prior art fuse-resistor protective devices are suitable for protecting certain types of cir-' cuits, they do not operate satisfactorily in other types of circuits. One reason for this is that the series resistor in such a device is responsive only to overcurrents and does not furnish any protection against overvoltage conditions. Another reason is that the abovementioned increase in the overall line impedance requires the fusible element in the device to have a higher resistance than would be necessary if the impedance of the resistor was not in series with the line circuit. This, in turn, requires the resistor to become very hot before the associated fusible element will melt. The heat generated by the resistor is particularly objectionable when the device is applied to a printed circuit board because the surface heat of the resistor is liable to burn or ignite the printed circuit board.

Accordingly, there is a need for a fuse-resistor which will be responsive to overvoltage conditions, which will not substantially increase the overall line impedance, and which will be designed in such a manner that its resistor will not become objectionably hot.

SUMMARY OF THE INVENTION The present invention is designed to provide a three terminal fuse-resistor device adapted for forming a dual element low impedance series fuse connection between the input portion and the load portion of an electric circuit and also for forming a resistive shunt path across the load portion of the circuit for acting as a voltage sensor. One fusible element has a high temperature resistance coefficient which causes it to melt in response to high overcurrents. The other fusible element has a low melting temperature and is adapted to melt in response to low overcurrents. Both elements are connected between a pair of terminals in series with the input and load portions of the circuit.

The protective device of this invention further comprises a resistor which is connected between a different pair of terminals so that it forms a shunt across the load for sensing an overvoltage condition. The resistor is also disposed in heat-radiating relation with respect to both of the fusible elements for effecting the melting of at least one of the fusible elements in response to an overvoltage condition. This detection of an overvoltage condition will function independently of the current in the line circuit because the connection of the resistor is such that the resistor is external to the load.

Since the resistor is not in series with the line circuit, its impedance is not added thereto and, for this reason, the two fusible elements each have a lower resistance than they ordinarily would have if the resistor had been connected in series with the line. Furthermore, the resistor will not become objectionably hot when overcurrent conditions occur in the electric circuit and, consequently, the protective device of this invention can be safely applied to a printed circuit board.

Thus, the elements of this protective device intercooperate to protect an electric circuit from abnormalities in both the voltage and the current in the electric power supplied thereto. This protection is provided by the interdependent functional relationship of the elements in the device without adding any substantial impedance to the line circuit. In other words, since the protective device of this invention employs dual fuse elements that are in series with a load and also employs a resistor that is in shunt with a load, the device presents a low series impedance to the circuit that is to be protected.

BRIEF DESCRIPTION OF THE DRAWING The features of this invention are more fully discussed hereinafter in connection with the following detailed description of the drawing in which:

FIG. 1 is a perspective view, partly in section, of a three terminal fuse-resistor protective device constructed in accordance with this invention;

FIG. 2 is a sectional side view of the protective device shown in FIG. 1; and I FIG. 3 is a schematic diagram of an electric circuit having the three terminal protective device of this invention connected therein.

DETAILED DESCRIPTION The invention will now be described with reference to the specific exemplary embodiment thereof which is shown in the drawing wherein it can be seen that the device is equipped with an input terminal 1, a load terminal 2, and a shunt terminal 3. These three terminals 1, 2, and 3 are made from a suitable electrically conductive wire, such as nickel or brass. The components of the device are mounted on both the exterior and interior of a hollow housing member constituted by a cylinder or tube 4 composed of an appropriate ceramic material.

The input terminal 1 is attached in any convenient manner to a cap 5 which fits snugly inside a socket 6 that is forcibly fitted inside the right end of the ceramic tube 4. Both the cap 5 and the socket 6 are made from a suitable electrically conductive material, such as brass. As is indicated in the drawing an end of the input terminal 1 is inserted through a hole in the cap 5 and is welded thereto. Thus, in this manner, the input terminal 1 is securely mounted at the right end of the tube 4. As is indicated in FIG. 3, this input terminal 1 is adapted to be connected in any suitable manner, such as by soldering, to the input portion 7 of an electric circuit 10 that is to be protected and which is to be understood as being mounted on a printed wiring board.

The output terminal 2 is positioned at the left end of he tube 4. This left end of the tube 4 is covered with an appropriate end cap 8 which is held securely on the tube 4 by means of a force fit. The end cap 8 is constructed from electrically conductive material, such as an alloy of nickel and iron. An end of the output terminal 2 is placed against the end cap 8 and is fastened thereto by a butt weld 9. The other end of the output terminal 2 is adapted to be connected, as by soldering, to one side of the load portion 11 of the circuit that is to be protected, as is illustrated in FIG. 3.

The shunt terminal 3 is formed with a right angular bend and has one portion placed in a narrow groove or slot 12 cut in the upper surface of the tube 4 and oriented in a direction parallel to the longitudinal axis of the tube 4. This portion of the terminal 3 is held within the slot 12 by any convenient means, such as by a coating 13 of a suitable adhesive. Another portion of the terminal 3 projects upward out of the right end of the slot 12 and is adapted to be connected, as by soldering, to that side of the circuit load portion 11 which is opposite the side to which the output terminal 2 is connected.

A thin, flat trigger plate 14 is positioned inside the tube 4 and is made of a suitable heat absorbing material, such as nickel. Attached to the plate 14 is a slow thermal rise element constituted by a lump 15 of solder alloy having a low melting temperature and which functions as low overcurrent protective means. A first portion of the fusible element constituted by the solder lump 15 serves to attach one end of a copper wire 16 to the trigger plate 14. The other end of the wire 16 is electrically connected to the end cap 8 by welding it thereto as is indicated by the reference numeral 17. Thus, the wire 16 functions as connecting means for completing an electrical path extending from the solder lump 15, over the wire 16, and then through the end cap 8 to the load terminal 2.

A second portion of the solder lump 15 serves to attach a portion of a second fusible element 18 to the trigger plate 14. This second fusible element 18 is constituted by an appropriate fuse wire which is a fast thermal rise element having a high temperature resistance coefficient. Since the wire 18 is adapted for generating sufficient heat for melting itself in response to high overcurrents, it function as high overcurrent protective means.

As can be seen in the drawing, the wire is folded back on itself for forming a loop which is placed under the solder lump 15. The ends of the wire 18 are fed through a hole in the socket 6 and are then electrically connected to an end of the input terminal 1 by welding it thereto as is indicated by the reference numeral 19. A glass fiber'sleeve 21 surrounds a portion of the fuse wire 18 for functioning as an arc suppressor to minimize any arcing which might occur when the wire 18 is melted by high overcurrents.

It can therefore be understood that, when either one of the fusible elements 15 or 18 melts, the electrically conductive path between the input terminal 1 and load terminal 2 will be broken. In order to increase the speed at which this electrically conductive path is opened, a coiled tension spring 22 is utilized. The main body of the spring 22 is inserted inside the tube 4. The turns at the left end of the spring 22 are enlarged so that they abut against the left side of the tube 4 to which they are securely attached by any convenient means, such as by soldering or welding them to the end cap 8.

The turns at the right end of the spring 22 are reduced in size and are forced over the left end of the v trigger plate 14. As is best seen in FIG. 2, the left end of the trigger plate 14 has protruding portions, or lugs, formed on each side thereof for securely holding those turns of the spring 22 that are forced thereover. Accordingly, when either of the fusible elements 15 or 18 melts, the resulting retraction of the tension spring 22 will effect the rapid opening of the electrically conductive path between the input terminal 1 and the load terminal 2.

The device of this invention further includes overvoltage protective means comprising a resistor 23 in the form of a suitable resistance wire that is wound around the outer surface of the tube 4. The process of winding the wire 23 is accomplished by folding the wire 23 back upon itself for forming a loop. The center of the looped portion of the wire 23 is passed around an anchor pin 24 which is inserted in a small hole drilled in the tube 4 and which is securely held therein by an appropriate adhesive. The bifilar portions of the wire 23 are then wound in a tapering manner around the tube 4. The end of one bifilar portion of the wire 23 is placed on the end cap 8 and is welded thereto. The end of the other bifilar portion of the wire 23 is welded to an anchor member 25 that is constituted by a bent end of the wire forming the shunt terminal 3. Thus, one end of the resistor 23 is connected through the end cap 8 to the load terminal 2, and the other end of the resistor 23 is connected through the anchor member 25 to the shunt terminal 3. This construction causes the connection of the resistor 23 to be such that the resistor 23 is external to the load 11 and in shunt therewith.

It can be seen in the drawing that this construction of the resistor 23 places the resistor 23 in heat-radiating relation with respect to both of the fusible elements 15 and 18. Accordingly, when an overvoltage condition occurs, the resistor 23 will generate heat which will be radiated to the fusible elements 15 and 18 for effecting the melting of at least one of them. In addition, heat from the resistor 23 is radiated to the trigger plate 14 which is adapted to absorb heat and which, in turn, both radiates and conducts its heat to the solder lump 15. As the solder lump 15 has a lower melting point than the fuse wire 18, the solder lump 15 will, in general, melt before the fuse wire 18. In In either case, the electrically conductive path between the input terminal 1 and the load terminal 2 will be opened and will thus provide the electric circuit 10 with protection against the overvoltage condition.

In this embodiment of the invention, it is desired to prevent the surface temperature of the resistor 23 from exceeding 400 F because, above this point, the heat generated by the resistor 23 might damage or ignite the printed wiring board on which the electric circuit 10 is mounted. In this regard, it should be noted that the surface temperature of the resistor 23 is a function of both current and time. Therefore, for any given value of overvoltage, the temperature of the resistor 23 will increase with time.

The time, or duration, of an overvoltage condition is shortened in accordance with this invention due to the resistor 23 being connected in shunt with the load 11. Because of this shunt connection, the impedance of the resistor 23 is not in series with the line circuit 7-11. Since the impedance of the resistor 23 is not added to the impedance of the line circuit 7-11, the two fusible elements 15 and 18 each have a lower resistance than would be required if the impedance of the resistor 23 had been added to the impedance of the line circuit 7-11.

For this reason, it is possible to employ an alloy for the solder lump 15 that has a melting temperature of 200 F and to so select the sizes of the lump 15, the trigger plate 14, and the resistor 23 as to cause the solder lump 15 to reach its melting temperature of 200 F at a point in time when the surface temperature of the resistor 34 is well below 400 F. During this overvoltage condition, the higher melting point of the fuse wire 18 prevents it from melting. The melting of the solder lump 15 allows the spring 22 to effect the opening of the line circuit 7-11 before the heat rises to a point which might cause damage to the associated printed circuit board.

In addition to this overvoltage protection, the electric circuit 10 is protected against low overcurrent conditions that may occur in the electric circuit 10. When a low overcurrent condition occurs, it will cause the solder lump and the fuse wire 18 to become hot. During this time, a certain amount of heat will be generated by the resistor 23 because it is in shunt with the load 11 and, as was explained above, this additional heat will be radiated to the trigger plate 14, the solder lump 15, and the fuse wire 18. Due to the proximity of the trigger plate 14 and the solder lump 15, the heat that is both generated and absorbed by the trigger plate 14 will be both radiated and conducted to the solder lump 15. Since the solder lump 15 has a low melting temperature, it will melt before the fuse wire 18 and will thus enable the spring 22 to effect the opening of the line circuit 7-11.

The electric circuit 10 is also protected against high overcurrents in a somewhat similar manner. When a high overcurrent condition occurs, it will cause the solder lump l5 and the fuse wire 18 to become hot, as was the case during a low overcurrent condition. Similarly, the additional heat generated by the resistor 23 will again be radiated to the solder lump 15, the trigger plate 14, and the fuse wire 18.

Although the solder lump 15 has a lower melting point than the fuse wire 18, the fuse wire 18 has a smaller mass and its temperature will rise more rapidly in response to the high overcurrent condition due to its internal wattage loss which is maximized by its high temperature coefficient of resistance characteristic. Itv

should be noted that, as was stated above, the fuse wire 18 has a lower resistance than would have been required if the impedance of the resistor 23 had been added to the impedance of the line circuit 7-11. Accordingly, the fuse wire 18 will melt before the solder lump 15 thereby permitting the spring 22 to effect the opening of the line circuit 7-11. Any arcing that might tend to be caused by the melting of the fuse wire 18 is prevented, as was stated above, by the glass fiber sleeve 21 which functions to absorb electric energy resulting from the rupture of the fuse wire 18.

From the above discussion, it can be understood that, whenever the electric circuit 10 is subjected to conditions of overvoltage, low overcurrents, or high overcurrents, the protective device of this invention will quickly function to open the line circuit 7-11 well before the particular abnormal condition can cause any damage to the circuit components or to the printed circuit board on which they are mounted.

What is claimed is:

1. In combination,

an electric circuit having an input portion and a load portion,

a three terminal fuse-resistor device adapted for protecting said electric circuit against abnormalities in electric power supplied thereto,

said device comprising a first terminal connected to said input portion,

a second terminal connected to one side of said load portion,

a third terminal connected to another side of said load portion,

overcurrent protective means responsive to overcurrent conditions in said electric power in said electric circuit,

overvoltage protective means including a resistor adapted for sensing an overvoltage condition in said electric circuit independently of current therein,

means for electrically connecting said overcurrent protective means to said first and second terminals,

and means for electrically connecting said resistor to said second and third terminals.

2. A combination in accordance with claim 1 wherein said overcurrent protective means comprise a first fusible element adapted to be effectively responsive to low overcurrents,

and a second fusible element adapted to be effectively responsive to high overcurrents.

3. A combination in accordance with claim 2 and further comprising means for electrically connecting said first and second fusible elements in series between said first and second terminals.

4. A combination in accordance with claim 2 and further comprising a heat absorbing member disposed in proximity with at least one of said fusible elements and adapted for transferring its absorbed heat to said element,

and supporting means adapted for supporting said resistor in heat radiating relation with respect to said heat absorbing member and at least one of said fusible elements.

5. In combination,

an electric circuit having an input portion and a load portion,

a three terminal fuse-resistor device adapted for forming a dual element low impedance series fuse connection between said input portion and said load portion of said electric circuit and also for forming a resistive shunt path across said load portion of said electric circuit for acting as a voltage sensor,

said device comprising a first terminal adapted for connection to said input portion of said electric circuit,

a second terminal adapted for connection to one side of said load portion of said electric circuit,

a third terminal adapted for connection to another side of said load portion of said electric circuit,

low overcurrent protective means comprising a first fusible element constituted by a slow thermal rise element having a low melting temperature,

first connecting means for electrically connecting a first portion of said first fusible element to said second terminal,

7 8 high overcurrent protective means comprising a secmeans for electrically connecting one end of said reond fusible element constituted by a fast thermal sistor to id second i l, rise element having a high temperature resistance and means for electrically connecting another end of coefficient and having a first portion thereof electrically connected to a second portion of said first fusible element,

said resistor to said third terminal, said resistor being disposed in heat-radiating relation with respect to both of said first and second fusible second connecting means for electrically connectlng a second portion of said second fusible element to elements whereby heat generated by said resistor in said first terminal response to said overvoltage is adapted for effectovervoltage protective means comprising a resistor 10 mg the melting of at least one of Said fusible adapted for generating heat in response to the aperitsplication of overvoltage thereto, 

1. In combination, an electric circuit having an input portion and a load portion, a three terminal fuse-resistor device adapted for protecting said electric circuit against abnormalities in electric power supplied thereto, said device comprising a first terminal connected to said input portion, a second terminal connected to one side of said load portion, a third terminal connected to another side of said load portion, overcurrent protective means responsive to overcurrent conditions in said electric power in said electric circuit, overvoltage protective means including a resistor adapted for sensing an overvoltage condition in said electric circuit independently of current therein, means for electrically connecting said overcurrent protective means to said first and second terminals, and means for electrically connecting said resistor to said second and third terminals.
 2. A combination in accordance with claim 1 wherein said overcurrent protective means comprise a first fusible element adapted to be effectively responsive to low overcurrents, and a second fusible element adapted to be effectively responsive to high overcurrents.
 3. A combination in accordance with claim 2 and further comprising means for electrically connecting said first and second fusible elements in series between said first and second terminAls.
 4. A combination in accordance with claim 2 and further comprising a heat absorbing member disposed in proximity with at least one of said fusible elements and adapted for transferring its absorbed heat to said element, and supporting means adapted for supporting said resistor in heat radiating relation with respect to said heat absorbing member and at least one of said fusible elements.
 5. In combination, an electric circuit having an input portion and a load portion, a three terminal fuse-resistor device adapted for forming a dual element low impedance series fuse connection between said input portion and said load portion of said electric circuit and also for forming a resistive shunt path across said load portion of said electric circuit for acting as a voltage sensor, said device comprising a first terminal adapted for connection to said input portion of said electric circuit, a second terminal adapted for connection to one side of said load portion of said electric circuit, a third terminal adapted for connection to another side of said load portion of said electric circuit, low overcurrent protective means comprising a first fusible element constituted by a slow thermal rise element having a low melting temperature, first connecting means for electrically connecting a first portion of said first fusible element to said second terminal, high overcurrent protective means comprising a second fusible element constituted by a fast thermal rise element having a high temperature resistance coefficient and having a first portion thereof electrically connected to a second portion of said first fusible element, second connecting means for electrically connecting a second portion of said second fusible element to said first terminal, overvoltage protective means comprising a resistor adapted for generating heat in response to the application of overvoltage thereto, means for electrically connecting one end of said resistor to said second terminal, and means for electrically connecting another end of said resistor to said third terminal, said resistor being disposed in heat-radiating relation with respect to both of said first and second fusible elements whereby heat generated by said resistor in response to said overvoltage is adapted for effecting the melting of at least one of said fusible elements. 